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.TH SEMGET "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
semget
\(em get set of XSI semaphores
.SH SYNOPSIS
.LP
.nf
#include <sys/sem.h>
.P
int semget(key_t \fIkey\fP, int \fInsems\fP, int \fIsemflg\fP);
.fi
.SH DESCRIPTION
The
\fIsemget\fR()
function operates on XSI semaphores (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.17" ", " "Semaphore").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fIsemget\fR()
function shall return the semaphore identifier associated with
.IR key .
.P
A semaphore identifier with its associated
.BR semid_ds
data structure and its associated set of
.IR nsems
semaphores (see
.IR <sys/sem.h> )
is created for
.IR key
if one of the following is true:
.IP " *" 4
The argument
.IR key
is equal to IPC_PRIVATE.
.IP " *" 4
The argument
.IR key
does not already have a semaphore identifier associated with it and
(\fIsemflg\fP &IPC_CREAT) is non-zero.
.P
Upon creation, the
.BR semid_ds
data structure associated with the new semaphore identifier is
initialized as follows:
.IP " *" 4
In the operation permissions structure
.IR sem_perm.cuid ,
.IR sem_perm.uid ,
.IR sem_perm.cgid ,
and
.IR sem_perm.gid
shall be set to the effective user ID and effective group ID,
respectively, of the calling process.
.IP " *" 4
The low-order 9 bits of
.IR sem_perm.mode
shall be set to the low-order 9 bits of
.IR semflg .
.IP " *" 4
The variable
.IR sem_nsems
shall be set to the value of
.IR nsems .
.IP " *" 4
The variable
.IR sem_otime
shall be set to 0 and
.IR sem_ctime
shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.IP " *" 4
The data structure associated with each semaphore in the set need not
be initialized. The
\fIsemctl\fR()
function with the command SETVAL or SETALL
can be used to initialize each semaphore.
.SH "RETURN VALUE"
Upon successful completion,
\fIsemget\fR()
shall return a non-negative integer, namely a semaphore identifier;
otherwise, it shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIsemget\fR()
function shall fail if:
.TP
.BR EACCES
A semaphore identifier exists for
.IR key ,
but operation permission as specified by the low-order 9 bits of
.IR semflg
would not be granted; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EEXIST
A semaphore identifier exists for the argument
.IR key
but ((\fIsemflg\fP &IPC_CREAT) &&(\fIsemflg\fP &IPC_EXCL))
is non-zero.
.TP
.BR EINVAL
The value of
.IR nsems
is either less than or equal to 0 or greater than the system-imposed
limit, or a semaphore identifier exists for the argument
.IR key ,
but the number of semaphores in the set associated with it is less than
.IR nsems
and
.IR nsems
is not equal to 0.
.TP
.BR ENOENT
A semaphore identifier does not exist for the argument
.IR key
and (\fIsemflg\fP &IPC_CREAT) is equal to 0.
.TP
.BR ENOSPC
A semaphore identifier is to be created but the system-imposed limit on
the maximum number of allowed semaphores system-wide would be
exceeded.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
Refer to
.IR "\fIsemop\fR\^(\|)".
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess
communication. Application developers who need to use IPC should
design their applications so that modules using the IPC routines
described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
A future version may require that the value of the
.IR semval ,
.IR sempid ,
.IR semncnt ,
and
.IR semzcnt
members of all semaphores in a semaphore set be initialized to zero when
a call to
\fIsemget\fR()
creates a semaphore set. Many semaphore implementations already do this
and it greatly simplifies what an application must do to initialize a
semaphore set.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fIftok\fR\^(\|)",
.IR "\fIsemctl\fR\^(\|)",
.IR "\fIsemop\fR\^(\|)",
.IR "\fIsem_close\fR\^(\|)",
.IR "\fIsem_destroy\fR\^(\|)",
.IR "\fIsem_getvalue\fR\^(\|)",
.IR "\fIsem_init\fR\^(\|)",
.IR "\fIsem_open\fR\^(\|)",
.IR "\fIsem_post\fR\^(\|)",
.IR "\fIsem_trywait\fR\^(\|)",
.IR "\fIsem_unlink\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.17" ", " "Semaphore",
.IR "\fB<sys_sem.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
